As tough cast iron, there are ductile cast iron obtained by adding Mg, Ca, Ce, and other elements of a graphite spheroidization agent and performing graphite spheroidization and compact vermicular cast iron (hereinafter referred to as “C/V cast iron”. Further, there is malleable cast iron obtained by heat treating white pig iron obtained by white pig casting.
In that C/V cast iron, the graphite does not become spheroidal and is present as an intermediate form of graphite masses etc. Further, malleable cast iron is good in castability and is rich in ductility and tough like with steel upon being heat treated, so is important as a material for machine structures. This malleable cast iron is classified into white heart malleable cast iron, black heart malleable cast iron, cast iron having a special base material, etc.
Among these, in black heart malleable cast iron, if leaving malleable cast iron castings as cast, they exhibit a white pig structure. This is hard and brittle, so in the production process, the iron is annealed for graphitization.
The time and temperature of the annealing conditions are determined based on numerous other casting factors, but usually this annealing includes two stages of annealing. The first stage annealing is performed at 900 to 980° C. of temperature over 10 to 20 hours. In this treatment, the free cementite is completely decomposed. The second stage annealing is performed by a combination of gradual cooling in a temperature range of 700 to 760° C. for the purpose of direct graphitization and long term treatment at 700 to 730° C. in range for graphitization of the cementite in the pearlite. In this way, the time required for the overall annealing process is usually 20 to 100 hours or so as described in the Iron and Steel Institute of Japan, 3rd Edition, Tekko Binran, Vol. V. “Casting, Forging, and Powder Metallurgy”, pp. 115 to 116, 1982.
Ductile cast iron and malleable cast iron can be rolled to a certain extent. Rolling cast semi-finished products to obtain cast iron plate, cast iron sheet, cast iron bars, and other rolled cast iron can be expected to open up uses for diverse applications. However, such cast iron has narrow rolling conditions and its applications are limited.
Further, as the method for obtaining the cast semi-finished products serving as the rolled materials, usually the casting method of pouring melt into a sand or other mold to obtain cast semi-finished product has been used, but sometimes continuous casting is performed as a means for raising productivity.
However, in the method of the above reference, there is the problem that with a malleable cast iron casting, a long time is required for the graphitization, so the productivity is remarkably poor and, further, the long heating results in oxidation and decarburization of the surface, so heating in a nonoxidizing atmosphere is required to suppress this and the treatment costs rise. Further, despite the annealing cycle being appropriate, the graphite precipitated after the treatment is not spheroidal. Therefore, this cannot be said to be graphitization providing sufficiently satisfactory characteristics. In particular, in terms of the balance of strength and ductility and the fatigue strength, malleable cast iron is not that superior compared with the usual rat cast iron. Further improvement from these characteristics is therefore desired.
As opposed to this, Japanese Patent Publication (A) 7-138636 does not describe a method for treatment for graphitization in a short time, and the graphite precipitating after treatment is not completely spheroidal. Further, with cast iron obtained by rolling ductile cast iron or malleable cast iron, the graphite forms thin flakes distributed in a laminar form at the time of rolling, so the workability ends up becoming poor.
Further, in continuous casting of usual cast iron, graphite molds are used for the purpose of prevention of chill, but white cast iron is difficult to continuously cast due to the wide region of copresence of the solid and liquid phases. As shown in Japanese Patent No. 4074747, therefore, this is not performed much at all.
In this way, as shown in Japanese Patent No. 3130670, using a twin-roll casting machine for white pig casting in sheets, and heat treating the result to produce cast iron sheets comprised of malleable cast iron is also conceivable as a method of production of tough sheets of cast iron, but in this case, in the same way as the case of production of malleable cast iron, the result becomes graphite masses, i.e., the spheroidization of the graphite is insufficient, so there is the problem of insufficient workability.